A known type of hydraulic brake apparatus is disclosed in Japanese Patent Application Laid-Open Publication No. 8(1996)-230634. This apparatus includes wheel brakes for applying a braking force to road wheels of the vehicle in response to hydraulic pressure supplied to the wheel brakes, a reservoir for storing brake fluid, a master cylinder which pressurizes the brake fluid supplied from the reservoir in response to the operation of a brake pedal and supplies the pressurized brake fluid to the wheel brakes, and an electromotive hydraulic pump which pressurizes the brake fluid supplied from the reservoir through the master cylinder and discharges the brake fluid to the wheel brakes.
The apparatus further includes a first hydraulic passage for supplying pressurized brake fluid from the master cylinder to the wheel brakes without passing through the hydraulic pump, a second hydraulic passage for supplying brake fluid from the master cylinder to the suction side of the hydraulic pump, a first electromagnetic valve interposed in the first hydraulic passage for opening and closing the first hydraulic passage, and a second electromagnetic valve interposed in the second hydraulic passage for opening and closing the second hydraulic passage. A third hydraulic passage supplies the brake fluid discharged from the hydraulic pump to a portion of the first hydraulic passage between the first electromagnetic valve and the wheel brakes, and a third electromagnetic valve is interposed between the wheel brakes and a connecting portion between the first hydraulic passage and the third hydraulic passage. A fourth hydraulic passage discharges brake fluid from the wheel brakes to the suction side of the hydraulic pump, and a fourth electromagnetic valve is interposed in the fourth hydraulic passage.
In the normal operation of the apparatus, the second and fourth electromagnetic valves are closed and the first and third electromagnetic valves are open. Further, the operation of the hydraulic pump is stopped. Therefore, when the driver of the vehicle depresses the brake pedal, the master cylinder pressurizes the brake fluid supplied from the reservoir and supplies the pressurized brake fluid to the wheel brakes through the first and third electromagnetic valves. Accordingly, the hydraulic pressure in the wheel brakes is varied in response to variations in the depressing force of the brake pedal.
In situations requiring that the brake apparatus be operated to produce higher hydraulic pressure in the wheel brakes than the hydraulic pressure generated by the master-cylinder when the brake pedal is depressed, the first electromagnetic valve is closed and the second electromagnetic valve is opened. Further, the hydraulic pump is driven by an electric motor. The hydraulic pump thus further pressurizes the pressurized brake fluid supplied from the master cylinder through the second electromagnetic valve and discharges the brake fluid to the wheel brakes through the third electromagnetic valve.
Further, in situations requiring that the apparatus be operated to supply hydraulic pressure to the wheel brakes when the brake pedal is not depressed, the first and fourth electromagnetic valves are closed and the second and third electromagnetic valves are opened. Further, the hydraulic pump is driven by the electric motor. The hydraulic pump thus pressurizes the brake fluid supplied from the reservoir through the master cylinder and the second electromagnetic valve, and discharges the brake fluid to the wheel brakes through the third electromagnetic valve.
The third and fourth electromagnetic valves are used for decreasing, re-increasing or maintaining the wheel brake hydraulic pressure supplied from the master cylinder or the hydraulic pump.
In the above-described brake apparatus, during operation of the hydraulic pump, a pulsation of the hydraulic pressure is generated at the suction side of the hydraulic pump due to the operation of the hydraulic pump. In situations where the master cylinder does not generate hydraulic pressure due to the non-depression of the brake pedal, this pulsation does not increase until a level at which problems are caused. However, in situations where the master cylinder generates hydraulic pressure due to depression of the brake pedal, the pulsation of the hydraulic pressure increases because the generated hydraulic pressure of the master cylinder acts on the suction side of the hydraulic pump. As a result, brake pedal vibration is caused and this gives the driver an unpleasant feeling. Further, vibration of hydraulic conduits or piping constituting the hydraulic passages is caused and noise is generated by this vibration. It might be possible to reduce the vibration of the brake pedal and hydraulic conduits by decreasing the cross sectional area of the second hydraulic passage so that the transmission of the pulsation to the master cylinder is suppressed. However, this would also increase the resistance for auctioning of the hydraulic pump and so the amount of brake fluid discharged from the hydraulic pump would be decreased when the brake pedal is not depressed.
In light of the foregoing, it would be desirable to provide an improved hydraulic brake apparatus for a vehicle which is not as susceptible to the foregoing drawbacks and disadvantages.
A need thus exists for a vehicle hydraulic brake apparatus that is able to reduce or prevent vibration of the brake pedal and hydraulic conduit during operations when the brake pedal is depressed without also decreasing the amount of brake fluid discharged from the hydraulic pump during operations involving non-depression of the brake pedal.